Role of the immune system in the pathogenesis of Hepatitis B. How HBV evades the immune system? What is the role of e-antigen and what is immune decoy
Immunopathogenesis of HBV
Like any other cell of our body, infectious hepatocytes have to process and present the proteins that are being synthesized inside them on their special MHC1 proteins. Normal cells will present normal proteins on MHCI as they produce normal body proteins while virus-infected cells will obviously present the viral proteins.
Not the whole proteins, rather fragments of the proteins are presented. Different fragments of several different viral proteins will be presented onto several different MHC1 receptors.
When a cell is infected with the virus it is like a hijacked factory. The factory that was producing essential proteins for the human is now forced to produce virus particles.
Millions of CD8+ Cytotoxic T cells having specificities for millions of different protein fragments are hungrily wandering in the blood in search of their prey. As soon as a specific Cytotoxic T cell comes across and encounters this infected liver cell, it realizes that this factory has been hijacked. The protein that it is presenting on MHC1 is not the endogenous human protein but a foreign protein. In this way, the cell comes to know that this cell is infected. As it can not enter inside the cell and destroy the virus, it does something horrible but that is need of the hour.
It destroys the whole cell. It throws special molecular grenades called Granzymes into pores that it forms into the cell or it can induce apoptosis, the programmed cell death, to the cell.
In this way, few virus-infected cells or virus-creation-factories are destroyed. These poor hijacked cells have to sacrifice themselves for the benefit of their friends; the remaining healthy cells. This process of killing viral-infected cells is called “Cell-mediated immune response” and it is crucial to contain the infection.
But killing the factories does not end the problem. The virus particles that have already been created by these factories are still strolling freely and happily. If not caught, they can continue to infect other cells. They will hijack other healthy cells and convert them into the infected cells. Those infected cells will create more virus particles. Those infected cells will then again killed by cytotoxic T cells and in this way maybe the whole liver will be destroyed by the immune cells, in the efforts to contain the virus. That’s how fulminant liver failure occurs.
So, how can we manage the virus particles that have already been released? Well, for this, antibodies are required.
Antibodies against which component of virus? Antibodies against which viral protein?
Antibody to capsid (core) protein? No, because it is packed inside the envelope. Antibody against capsid cannot reach to attach with the capsid.
Antibody against the surface proteins is required. It will attach to the surface proteins of HBV and prevent their entry into the healthy cells. Furthermore, these antibodies-tagged virus particles will be cleared by macrophages quickly. This antibody-mediated response is called “Humoral immune response”.
So, in order to control the infection body needs a dual response that must be quick and draconian:
The extent of disease and its symptomatology depends upon the efficiency of the immune response.
If the immune response is very quick, only a few hepatocytes will be infected and destroyed before the Anti-HbsAb are created and prevent the entry of the virus into the remaining healthy hepatocytes. In such cases, the patient may even be asymptomatic.
While if the response is delayed more and more cells will be infected and will be destroyed by cytotoxic T cells leading to full-fledge hepatitis.
It has been observed that the virus does not cause a direct cytopathic effect, hepatitis (inflammation of hepatocytes) occurs when immune cells attack the virus-infected cells.
Actually, most of the time, it is not so easy for the body to clear the infection. Virus fools both arms of the immune system. (Cell-mediated and Humoral).
Hummoral Immune Response
As the immune system creates antibodies against the surface proteins they should bind with the virus particles and prevent the entry of the virus into the healthy hepatocytes.
But antibodies do not always bind the surface proteins of infectious virus particles. Most of the time, antibodies bind with non-infectious sub-viral particles, which turns out to be a useless activity.
As it binds with these non-infectious particles, the infectious Dane particles slip away from the immune system and continue infecting the cells. This is called “Immune decoy”. Subviral particles because the resembles the actual viral particles, trick the antibodies, and they bind with them thinking that this may be the actual virus.
As these are 1000x to 10000x higher in number than the actual virus, antibodies are 1000-10000x more likely to bind with these particles rather than the real virus.
Imagine if HBV never created such subviral particles, how easy it would be for the immune system to clear the infection. Just kill the few infected cells and create and attach antibodies to surface proteins of the “real” virus particles and that’s it.
So, in order to effectively catch the virus, antibodies must be created in much larger quantities. Antibodies against surface proteins must be in excess to the viral and subviral particles to be detectable in the lab. Virus-bound or subviral particle-bound antibodies are undetectable in the lab, that’s why AntiHbsAb are detected late in the course of infection.
If antibodies are created in excess, they are protective. They can bind all the virus and subviral particles, meanwhile, cytotoxic T cell clears the infected cells and destroy everything present inside.
Cell-mediated Immune Response
HBV is far more clever. The story of it’s evil-mindedness does not end here. It befools the cell-mediated immune response as well.
To understand that, let's learn how cell-mediated and the antibody-mediated immune response is generated.
As the virus particles come out of the infected cell. They are taken up by the innate immune cells. For example, Macrophages. Macrophages have special receptors that can recognize, through a particular pattern, the microbial particles. Although macrophages are the part of the innate immune system that has no memory of previous exposure to any microbe, these receptors are so “great” and “awesome” that they still help macrophages to recognize the microbes.
They were first discovered in Drosophila. These receptors also have a role in embryonic growth as well. Scientists studying them were so much fascinated by these receptors that they named it “great” receptors, or Toll-receptors which is a German word that literally means “Great”.
Receptors similar to “Toll” were discovered in human macrophages so they are named “Toll-like receptors” (TLRs).
So with these TLRs, macrophages recognize the virus particles, engulf and process them and preset their fragments or epitopes on their MHCII (not MHCI). MHCII are present on professional antigen-presenting cells.
As they do so, Helper T cells or immature CD4+ cells recognize the antigen bound-MHCII and attach it with them. Meanwhile, the CD8+ cytotoxic T cells have attached the antigen bound-MHCI of the infected hepatocyte. They still are not active and can not kill the cells.
Remember those e-antigens produced by the infected cells. They will be caught by the B cells, processed and presented on B-cells’s MHCII. They will also be recognized by the helper cells.
CD4+ cell when attaches itself with Macrophage presenting the antigen gets activated. It can now be transformed into Helper type I cells that promote cell-mediated immune response or into Helper Type II cells that promote antibody-mediated immune response.
Anigen-primed CD8+ cells release IL-10 the chemical messengers that cause Th1 (helper type I) differentiation. While B cells cause Th2 differentiation.
Th1 cells will generate such interleukins that cause the clonal proliferation (making several copies) of antigen primed CD8+ cells and their activation. Thus they will effectively kill the infected cells.
Th2 cells will generate interleukins that cause B cell stimulation and release of antibodies.
The cell-mediated immune response is necessary to kill the virus generating factories.
But a large number of e-antigen secreted by them will cause differentiation into Th2 cells rather than Th1, which will lead to an anti-HbeAb generation rather than activation of cytotoxic T cells. This is
another immune decoy mechanism that decreases cell-mediated immune response.
This delays the clearance of the virus and it may lead to chronic hepatitis as the virus will be cleared very slowly due to weak cytotoxic T cell response.
e-antigen was also found to downregulate these TLRs on the macrophages thus further decreasing the immune response.
In such a situation the virus will only be cleared after AntiHbeAb are created in excess. The excess anti e antibody will catch the antigen that can no more stimulate a further Th2 cell line generation. Now only the cytotoxic T cell will be there to promote Th1 cell-line. Th1 cells will activate the cytotoxic T cells and the infection will be cleared.
If e-antigen is found in blood it means the virus is enjoying the luxury of living inside the cell and procreating actively. If e-antigen is not found it means the virus is not actively replicating and it indicates a good prognosis.
Sometimes, the virus’s DNA remains inside the nucleus of the human cells as non-chromosomal DNA (episome), producing virus particles very slowly.
Clinical features and Pathogenesis:
In the previous lecture, we discussed how symptomatology and extent or severity of disease depends upon the number of hepatocytes that are killed before immune system overpowers the infection and kills the virus.
As the immune cells are stimulated they produces cytokines, the chemical messengers, that produce non-specific symptoms of inflammation.
Fever due to increase in temperature set point
Nausea, vomiting, lethargy.
Liver strategically placed in such a way that blood from GIT as well as from the spleen reaches the liver by hepatic portal vein. Here, the liver detoxifies it.
Spleen, the graveyard of RBCs, destroys the old RBCs. Splenic macrophages convert the heme into bilirubin and release It into the blood. Bilirubin being a hydrophobic compound may aggregate here, so albumin is used to transport it within the blood, an aqueous medium, towards the liver.
Bilirubin enters the hepatocytes along the concentration gradient. In the hepaotocytes, 2 molecules of Glucuronic acid are attached to the bilirubin, thus forming conjugated bilirubin (Bilirubin Diglucuronide).
Conjugated bilirubin is then pushed into the biliary canaliculi against the concentration gradient, which is ultimately transported to the intestine.
These two processes, conjugation and secretion into biliary canaliculi are active processes. Weak, inflammaed and damaged hepatocytes can not perform this task effectively. Generally, the conjugation ability of liver is much more than routinely required. So, conjugation can occur to some extent unless most of the liver parenchyma is damaged however the damaged hepatocte can not excrete out this conjugated bilirubin into the biliary canalculi. So conjugated bilirubin builds up and it is then sent back into circulation. Furthermore, because conjugation is not very much effective, unconjugated (free) bilirubin also accumulates. So, this will lead to mixed hyperbilirubinemia but conjugated bilirubin predominates.
Bilirubin can accumulates into different parts of the body.
In the skin and eyes. Which will manifest as yellowing of sclera and skin. This clinical sign is called “Jaundice”
For immunization we give surface antigen only which will lead to production of anti-surface antibody that is protective. So in an immunized person only this antibody will be detectable.